Abstract
The charge characteristics of montmorillonite have significant effects on its hydration and application performances. In this study, a molecular dynamics simulation method was used to study the influence of the charge position and charge density of montmorillonite on the distribution of H2O and Ca2+ in layers. The results showed that when the layer charge is mainly derived from the substitution among ions in the tetrahedron, a large number of Hw and Ot are combined into a hydrogen bond in the interlayer, thus the water molecules are more compactly arranged and the diffusion of water molecules among the layers is reduced. In addition, the Ca2+ are diffused to the sides by a concentrated distribution in the central axis of the layer. As the charge density of the montmorillonite increases, the polarity of the Si–O surface increases, which lesds to the deterioration of the diffusibility of the water molecules and the structure of the water molecules in the interlayers is more stable. The increase in the layer charge density lesds to the expansion of the isomorphic substitution range of the crystal structure, which results in a more dispersed distribution of Ca2+ among the layers under the action of electrostatic attraction between the substituted negative sites and the Ca2+.
Highlights
Montmorillonite is a typical clay mineral with a unique crystal structure, which provides it with a high cation exchange capacity and high expansion capacity, it is widely used in various fields.The molecular simulation method used in this study is useful and has several advantages [1,2,3]
Can it simulate the molecular structure of the material [4], it can simulate the dynamic change of molecules [5,6], intuitively describe the mechanism of the chemical reaction at the molecular and atomic scale, and verify the rationality of conclusions or predict the results of experiments [7,8]
Mary et al used the Monte Carlo method and the molecular dynamics method to study the Na-MMT and Cs-MMT, and compared the structure and dynamics characteristics with the experimental data, the results showed that the calculated layer distance and the diffusion coefficient of water molecules in single hydrate and interlayer cations were basically identical by the experimental data [14]
Summary
Montmorillonite is a typical clay mineral with a unique crystal structure, which provides it with a high cation exchange capacity and high expansion capacity, it is widely used in various fields. Mary et al used the Monte Carlo method and the molecular dynamics method to study the Na-MMT and Cs-MMT, and compared the structure and dynamics characteristics with the experimental data, the results showed that the calculated layer distance and the diffusion coefficient of water molecules in single hydrate and interlayer cations were basically identical by the experimental data [14]. Mary and Mignon used the Monte Carlo method and the molecular dynamics method to study the behavioral characteristics of different cations (Li+ , Na+ , and K+ ) in the process of montmorillonite hydration, and found that with the increasing of the montmorillonite interlayer water content, Na+ and Li+ were easy to separate from the interlayer of montmorillonite, while K+. In this study, according to the layer charge density distribution characteristics of Ca-montmorillonite in nature, we used molecular simulation to establish the montmorillonite crystal structure models with different layer charge characteristics, and used molecular dynamics as a powerful computational tool to study the influence mechanism of montmorillonite layer charge density and inter-ion substitution position on the distribution characteristics of water molecules and Ca2+ in the interlayer
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